1. Field of the Invention
This invention relates to minimizing data overflow in communications links. More specifically, the invention relates to arrangements for minimizing data overflow by managing data buffer occupancy, especially in a Fibre Channel (FC) environment.
2. Related Art
Fibre Channel (FC) technology is well known in the art. See, for example, Chapter 13.5 (“Fibre Channel”) of William Stallings' Data and Computer Communications (Prentice-Hall, 1997), which like all documents cited herein, is incorporated by reference.
Fibre Channel (FC) provides a credit based flow control to protect against collisions and assure that the receiving port is not flooded with more data than it can handle. This approach avoids overrun and also provides a way to mitigate performance degradation over distance by allowing more in-flight frames. However, Fibre Channel has a detrimental impact on performance if extended distances separate source and destination devices.
Storage area network (SAN) users have deployed Fibre Channel Extender devices to minimize the performance degradation over extended distances. However, such devices, if not properly designed, can actually have adverse impact on system performance. For example, under various realistic conditions, channel extenders drop packets.
One approach involved use of a supplemental overflow data channel. For example, U.S. Patent Application Publication No. 2001/0024432 (Zehavi et al.) discloses an arrangement in which, when a data rate of a packet exceeds a capacity of a main channel, the packet is also transmitted on an overflow channel.
Other approaches have involved complex, distributed management schemes. For example, U.S. Patent Application Publication No. 2003/0065736 (Pathak et al.) discloses an arrangement in which nodes in a wireless data network keep track of an amount of memory that is reported to be available in a client device, so that a network essentially ensures that overflow does not occur in the client devices.
Another approach involves frame pull flow control in which frames remain in a first Fibre Channel device until they are requested by a second Fibre Channel device; see U.S. Patent Application Publication No. 2003/0202474 (Kreuzenstein et al.).
One approach to extending fibre channel performance range is disclosed in U.S. Patent Application Publication No. 2003/0227874 (Wang), which involves a supplemental buffer arrangement governed by a locally generated ready signal. The locally generated signal is substituted for the ready signal that would be remotely generated according to the Fibre Channel standard. Wang's transmitting node keeps a count of the remote buffer usage and stops sending frames if the remote buffer is full. This count is incremented when transmit node sends a frame to the remote node and is decremented when it receives a R_RDY (receiver ready) signal. Undesirably, such arrangements suffer performance degradation if the buffer at the remote node is less than a certain size, often owing to the effects of latency (round-trip communication delay) when awaiting R_RDY signals. Such performance degradation can persist even if there is no data rate mismatch. Most Fibre Channel extenders, including the one disclosed by Wang patent, perform optimally if they operate within design parameters. However, as latency is increased beyond design values, performance decreases and usable bandwidth is wasted.
Accordingly, there is a need in the art for arrangements that adapt to increased latency or network impairment and still provide an optimal performance. Also, there is a need in the art for an arrangement that intelligently and transparently minimizes or eliminates data overflow, even over long distances and using Fibre Channel technology, thus allowing fulfillment of quality of service (QoS) guarantees. There is also a need in the art for an approach that minimizes dropped traffic to an insignificant amount, and, further, that is generic enough to adapt to all data rates and distances between the source and destination devices.